Ball screw

ABSTRACT

Ball screw having a spindle nut ( 2 ) arranged on a spindle ( 1 ), together with balls ( 3 ) which are arranged in a series and are capable of rolling in a helical ball track ( 4 ), the helical ball track ( 4 ) being defined by a thread groove ( 5 ) formed on the spindle ( 1 ) and by a thread groove ( 6 ) formed on the spindle nut ( 2 ), the balls ( 3 ) arranged in a series being returned in a return duct ( 7 ) from an outlet end to an inlet end of the helical ball track ( 4 ), the divided circle end play of the series of balls being less than or equal to 0.3 millimeters or the ratio between the pitch of the helical ball track ( 4 ) and the diameter of the balls ( 3 ) ranging from 1.02 to 1.2 or the thread grooves ( 5, 6 ) of the spindle nut ( 2 ) and of the spindle ( 1 ) in each case having a gothic profile ( 8, 9 ), a pressure angle formed by the balls ( 3 ) in contact with the gothic profile ( 8, 9 ) ranging from 15 to 35 degrees.

The present invention relates to a ball screw. The invention relates,for example, to those ball screws which have recently come to be used inelectro-mechanical steering mechanisms of motor vehicles. In suchapplications the spindle nut may be driven by an electric motor, thespindle being displaced linearly under the rotation of the spindle nut.

DE 101 40 884 A1, for example, has disclosed a ball screw according tothe features of the preamble of claim 1. In such ball screws runningnoises occur in the event of especially rapid rotational movements ofthe spindle nut. These running noises are often caused by abrupt contactbetween the balls and also by knocking together of the balls in thereturn duct of the spindle nut. Such running noises are perceived as anuisance especially where such ball screws are used in electromechanicalsteering mechanisms. The object of the present invention is therefore tospecify a ball screw according to the features of the preamble of claim1, in which running noises are reduced.

The invention proposes various measures for achieving this object, whichnot only independently of one another but also in combination with oneanother significantly reduce the running noises. In a first measureaccording to the invention the divided circle end play (TES) of theseries of balls is less than or equal to 0.3 mm. The balls are arrangedin a series, the series of balls circulating endlessly in their helicalball track by way of the return duct. It has been discovered that therunning noises can be significantly reduced if the distance between thefirst and the last ball in the series of balls—that is to say thedivided circle end play (TES)—is less than or equal to 0.3 mm. With thisdivided circle end play, although knocking contacts between the ballscannot be excluded, the pulses occurring here are only faintly manifest,owing to the small divided circle end play.

In a further measure according to the invention the ratio between thepitch of the helical ball track and the diameter of the balls rangesfrom 1.02 to 1.2. This ratio permits the use of the largest possibleballs. Large balls run more quietly in the ball screw than balls of asmaller diameter. This measure likewise helps to reduce the runningnoises. The proposed range is particularly useful in ensuring quietrunning.

In a further measure according to the invention the thread grooves bothof the spindle nut and of the spindle in each case have a gothicprofile, a pressure angle formed by the balls in contact with the gothicprofile ranging from 15 to 35°. These small pressure angles result inshallow thread grooves, so that only a slight lifting of the balls isnecessary, especially in the return area of the balls. Harmonious ballguiding is consequently possible even in the ball return area. Theoverall height of the spindle nut can furthermore be reduced. In thereturn area the balls are lifted off from the thread groove of thespindle and are introduced into the return duct. It was found with theproposed pressure angle range that running noises are significantlyreduced in comparison to other pressure angles. A common feature of allthese measures is that they are purposely intended to influence the pathof the balls and the ball movements.

The return duct is preferably defined by a return groove of the spindlenut, this return groove likewise having a gothic profile, which alsopreferably has a pressure angle ranging from 25 to 35°. The returngroove may be conventionally formed on a return piece, which is arrangedin a recess in the spindle nut.

The invention will be explained in more detail below with reference toan exemplary embodiment represented in a total of three figures, ofwhich

FIG. 1 shows a longitudinal section through a ball screw according tothe invention,

FIG. 2 shows a cross-section through the ball screw and

FIG. 3 shows an enlarged representation of a partial longitudinalsection through the ball screw according to the invention.

The ball screw according to the invention represented in FIGS. 1 to 3comprises a spindle nut 2 arranged on a spindle 1, together with balls 3which are arranged in a series and are capable of rolling in a helicalball track 4. The helical ball track 4 is defined by a thread groove 5formed on the spindle 1 and by a thread groove 6 formed on the spindlenut 2. The balls 3 arranged in a series are returned in a return duct 7of the spindle nut 2 from an outlet end to an inlet end of a common turnof the helical ball track 4.

FIG. 2 clearly shows the divided circle end play (TES) between the firstand the last ball of a series of balls. According to the invention thisdivided circle end play is less than or equal to 0.3 mm. In the figurethis divided circle end play is shown somewhat exaggerated merely forthe reasons of clarity.

It can be seen from FIG. 3 that the thread grooves 5 and 6 in thespindle 1 and the spindle nut 2 are each provided with a gothic profile8 and 9 respectively. This gothic profile permits a geometricallyprecise guiding of the balls along the helical ball track 4. Thepressure angle a formed by the balls 3 in contact with the gothicprofile 8, 9 of the thread grooves 5, 6 preferably ranges from 25 to35°. The ratio between the pitch P and the diameter of the balls 3furthermore ranges from 1.02 to 1.2.

It can be seen from FIGS. 1 and 2 that a return piece 10 is arranged inthe return area of the balls 3. This return piece 10 has a return groove11 defining the return duct 7 and on which the balls 3 roll. This returngroove 11 is likewise provided with a gothic profile (not representedfurther), as is also provided in the thread grooves 5 and 6 of thespindle 1 and the spindle nut 2. The gothic profile of this returngroove also has a pressure angle formed in contact with the balls whichranges from 15 to 35°.

A distinguishing feature of the ball screw depicted and described hereis its particularly quiet running. This ball screw according to theinvention is therefore especially suited to use in electromechanicalsteering mechanisms.

Instead of the ball screw with internal return described here by way ofan example, it is also possible to provide a ball screw with an externalreturn by arranging a tubular return piece radially outside the threadgroove of the spindle nut.

REFERENCE NUMERALS

-   1 Spindle-   2 Spindle nut-   3 Ball-   4 Helical ball track-   5 Thread groove-   6 Thread groove-   7 Return duct-   8 Gothic profile-   9 Gothic profile-   10 Return piece-   11 Return groove

1. A ball screw having a spindle nut (2) arranged on a spindle (1),together with balls (3) which are arranged in a series and are capableof rolling in a helical ball track (4), the helical ball track (4) beingdefined by a thread groove (5) formed on the spindle (1) and by a threadgroove (6) formed on the spindle nut (2), the balls (3) arranged in aseries being returned in a return duct (7) from an outlet end to aninlet end of a common turn of the helical ball track (4), characterizedin that the divided circle end play of the series of balls is less thanor equal to 0.3 millimeters.
 2. A ball screw having a spindle nut (2)arranged on a spindle (1), together with balls (3) which are arranged ina series and are capable of rolling in a helical ball track (4), thehelical ball track (4) being defined by a thread groove (5) formed onthe spindle (1) and by a thread groove (6) formed on the spindle nut(2), the balls (3) arranged in a series being returned in a return duct(7) from an outlet end to an inlet end of a common turn of the helicalball track (4), characterized in that the ratio between the pitch of thehelical ball track (4) and the diameter of the balls (3) ranges from1.02 to 1.2.
 3. A ball screw having a spindle nut (2) arranged on aspindle (1), together with balls (3) which are arranged in a series andare capable of rolling in a helical ball track (4), the helical balltrack (4) being defined by a thread groove (5) formed on the spindle (1)and by a thread groove (6) formed on the spindle nut (2), the balls (3)arranged in a series being returned in a return duct (7) from an outletend to an inlet end of the helical ball track (4), characterized in thatthe thread grooves (5, 6) of the spindle nut (2) and of the spindle (1)in each case have a gothic profile (8, 9), a pressure angle formed bythe balls (3) in contact with the gothic profile (8, 9) ranging from 15to 35 degrees.
 4. The ball screw as claimed in claim 3, characterized inthat the return duct (7) is defined by a return groove (11) which isprovided on the spindle nut (2) and on which the balls (3) roll, thereturn groove (11) having a gothic profile.
 5. The ball screw as claimedin claim 3, characterized in that a pressure angle formed by the balls(3) in contact with the gothic profile of the return groove (11) rangesfrom 25 to 35 degrees.
 6. The ball screw as claimed in claim 4,characterized in that the return groove (11) is formed on a return piece(10), which is arranged in a recess in the spindle nut (2).